The invention relates to current interrupters of vacuum type for use in controlling fault currents associated with transmission lines in power distribution systems.
Enormous increases in electrical power demand has led utility systems to use ever higher voltages in the transmission of power. Fault currents due, for example, to ground shorts can rapidly become enormous on such high voltage lines. Therefore, as transmission voltages rise there is a continuing need in the electric power industry for improved current interrupting and limiting devices capable of rapidly limiting fault currents which would otherwise seriously damage equipment.
Current interrupters are used to halt, or with a parallel current-suppressive load, to limit excessive currents customarily associated with faults. Interrupters of the vacuum type generally comprise a pair of relatively movable electrodes which can be placed in electrical contact to provide a free path for current flow. When excessive current is detected the contacts are separated, creating a voltage drop which generally induces arcing across the gap. The arc continues to carry substantially the full fault current. The arc must then be extinguished to prevent further current flow through the interrupter.
In the past it has been possible in alternating-current systems to permit the arc to burn until a current zero is reached, at which time the arc disappears and reignition prevented if the dielectric strength across the electrode gap is sufficient. In present high-voltage lines fault currents build up too rapidly to await even a single current half-cycle before extinguishing the arc. Instead, various techniques are employed to disrupt the current flow between electrodes immediately after separation. One technique is the application of a transverse magnetic field across the inter-electrode gap. The magnetic field tends to disrupt the arc so as to increase the voltage drop between the arcing electrodes. Such voltage drop will be sufficient either to break the flow of current or divert it into a parallel current-suppressive circuit which will limit the current below dangerous levels.
A problem encountered in magnetic arc suppression is local electrode overheating, particularly of the anode. The interelectrode gap immediately following separation is filled with a highly conductive neutral plasma comprising ions of cathode material and electrons. This plasma generally permits continuation of substantially the entire preseparation current flow with substantially no voltage drop. Ions emerge from the cathode at a multitude of infinitessimal emitting portions called "cathode spots". Application of a magnetic field causes these ions and associated electrons to be swept from between the electrodes greatly increasing the voltage drop across the arc and permitting current interruption. However, if there is overheating of the anode during arcing, it has been found that the anode also begins emitting conductive ions. Anodic ions emerge from "anode spots" which generate a conductive plasma similar to that from cathode spots, but which is far less susceptible to the magnetic field. Anode spots have been found to emit an ion-electron plasma in the form of a well collimated beam in which the ions emerge with sufficient energy to reach the cathode despite the magnetic field. This is thought to be due to the large fall region associated with the anode which permits rapid ion acceleration away from the anode. The arc emerging from the anode creates a permanent current path until the next current zero and is usually not extinguished by the magnetic field. Thus, successful rapid arc extinction is in part dependent upon reducing overheating of the anode and preventing anode spots.
It is known that relatively large electrode contact surfaces prevent bunching of the arc and hence reduce local overheating of the electrodes. To rapidly interrupt fault currents, however, it is necessary to separate the electrodes exceedingly rapidly, for example, 2 centimeters in one millisecond. Since large electrodes have a mass on the order of 2 kg or more, large separating forces are required, greatly increasing the cost of the interrupter.
Use of smaller mass electrodes with smaller contact surfaces tends to constrict the arc, this causes electrode overheating which leads to the development of anode spots and the failure of current interruption, as described above.
Use of a large fixed anode and small movable cathode greatly reduces the likelihood of anode spot formation. Unfortunately the polarity of voltage and current at the time of fault are unknown in alternating current applications.